Recently, against the background of worldwide efforts addressing the problems of global warming and energy, development of internal combustion engines with lower fuel consumption has been demanded in the field of vehicles as well, including automobiles and others. The most notable example is a lean-burn internal combustion engine. In particular, a direct injection internal combustion engine is more effective in reducing fuel consumption than the known intake-port injection internal combustion engine, because the former engine enables combustion to be performed at an air/fuel ratio of not less than 40 by directly injecting fuel in a cylinder to produce a stratified air-fuel mixture.
On the other hand, against recognition of environmental problems such as air pollution, development of internal combustion engines with low-pollution exhaust gases has also been demanded and specific exhaust emission control has been legislated in various countries over the world at standards becoming severer year by year. Because a three-way catalyst cannot clean NOx discharged from the internal combustion engine during a lean operation, the lean-burn internal combustion engine generally includes a lean NOx catalyst to clean NOx during the lean operation.
During the lean operation of the internal combustion engine, i.e., in an oxidizing atmosphere, the lean NOx catalyst stores NOx and oxidizes and cleans HC, CO. Also, to clean the stored NOx, the lean NOx catalyst has the function of desorbing the stored NOx and causing oxidation-reduction reactions between the NOx and a reductant (H2, CO and HC) discharged from the internal combustion engine when the internal combustion engine is operated in a mode of rich operation, i.e., in a reducing atmosphere.
That rich operation of the internal combustion engine is generally called a rich spike or rich spike control. During the rich spike, there occur not only deterioration of fuel economy, but also deterioration in cleaning of exhaust emissions if the oxidation-reduction reactions between the stored NOx and CO, HC discharged from the internal combustion engine are insufficient. It is therefore required to optimize a rich spike control process applied to the lean NOx catalyst from the viewpoints of both better fuel economy and improved cleaning of the exhaust emissions.
On the other hand, to be adapted for exhaust emission control legislated at standards becoming severer, as mentioned above, it has also become popular in the lean-burn internal combustion engine to separately dispose a three-way catalyst in addition to the lean NOx catalyst for the purposes of reducing the exhaust emissions at the startup and reducing the exhaust emissions during a stoichiometric operation.
The three-way catalyst is generally loaded with precious metals, i.e., Pd, Pt and Rh, and an oxygen storing agent, represented by Ce, with intent to increase cleaning efficiency during the stoichiometric operation. The oxygen storing agent has the function of storing oxygen in an oxidizing atmosphere and releasing the stored oxygen in a reducing atmosphere, and it serves to suppress deterioration in cleaning of the exhaust emissions by moderating a shift of the air/fuel ratio from the stoichiometric ratio within the three-way catalyst.
However, when the three-way catalyst is applied to the lean-burn internal combustion engine, oxygen is stored in the three-way catalyst during the lean operation. For example, in the case of the three-way catalyst being arranged upstream and the lean NOx catalyst being arranged downstream, therefore, the oxygen stored in the upstream three-way catalyst is released and oxidizes the reductant discharged from the internal combustion engine during the rich spike. For that reason, to introduce reducing gases to the lean NOx catalyst, the oxygen stored in the upstream three-way catalyst must be consumed.
In other words, the process of the rich spike is divided into two, i.e., a process of desorbing O2 stored in the three-way catalyst and a process of desorbing and cleaning the NOx stored in the lean NOx catalyst. As described later in detail, the inventors have conducted experiments on the rich spike in those two processes from the viewpoints of reducing a fuel consumption and improving the cleaning efficiency of the exhaust emissions, and have found that the optimum air/fuel ratio differs between those two processes. As described later, such finding, i.e., a difference in the optimum air/fuel ratio during the rich spike, depends on characteristics of the three-way catalyst and the lean NOx catalyst.
JP-A-2001-132440 as one example of the prior art discloses a technique of arranging a three-way catalyst in the upstream side, arranging a NOx-occluding and reducing catalyst in the downstream side, and setting a time for holding a stoichiometric state in the three-way catalyst after a rich spike to be not longer than 1 s. This technique focuses attention on an oxygen storing and desorbing characteristic of the upstream three-way catalyst and is intended to reduce NOx by effectively supplying a reductant to the downstream NOx catalyst when an operation mode is changed from rich to lean after the end of the rich spike. Stated another way, the disclosed technique is not intended to provide an optimum air/fuel ratio in each of the process of desorbing the oxygen stored in the three-way catalyst and the process of desorbing and cleaning the NOx stored in the lean NOx catalyst during the rich spike from the viewpoints of both better fuel economy and improved cleaning of the exhaust emissions.
Further, JP-A-2002-70611 discloses a technique of supplying a reductant in larger amount as the amount of oxygen stored in a three-way catalyst and a lean NOx catalyst increases. This technique is intended to optimize the spike time with attention focused on the need of a reductant to consume the stored oxygen. Stated another way, the disclosed technique is not intended to optimize the air/fuel ratio in the process of desorbing the oxygen stored in the three-way catalyst and the air/fuel ratio in the process of desorbing and cleaning the NOx stored in the lean NOx catalyst.
Still further, JP-A-2002-13414 discloses a technique of, in an internal combustion engine including a three-way catalyst and a NOx catalyst, supplying a reductant for desorbing NOx stored in the NOx catalyst and a reductant for cleaning the desorbed NOx in an independently controllable manner. However, this disclosed technique is also not intended to optimize the air/fuel ratio in the process of desorbing the oxygen stored in the three-way catalyst and the air/fuel ratio in the process of desorbing and cleaning the NOx stored in the lean NOx catalyst.
Thus, any of the techniques disclosed in the above-cited laid-open publications is not intended to optimize the air/fuel ratio in the process of desorbing the oxygen stored in the three-way catalyst and the air/fuel ratio in the process of desorbing and cleaning the NOx stored in the lean NOx catalyst from the viewpoints of both improved cleaning of the exhaust emissions and better fuel economy.
The present invention has been made in view of the above-mentioned problems in the art, and its object is to provide an internal combustion engine control device for, in a lean-burn direct-injection internal combustion engine provided with an exhaust emission cleaner including a three-way catalyst and a lean NOx catalyst, optimizing an air/fuel ratio during a rich spike from the viewpoints of both better fuel economy and improved cleaning of exhaust emissions.